opinion bita l The power to improve
BITA Technical Policy Committee member David Culshaw, the product & applications manager for Exide Technologies, explains how new battery charging technologies are heralding greater operational efficiencies and a safer, greener environment for materials handling.
he issue of battery charging is central to any operator of electric-powered industrial trucks. The choice of charging technology and charger has a big influence on factors such as cost and reliability. At present there are three principal types of charger available: Taper, Regulated and High Frequency chargers. Taper and Regulated chargers are what we describe as the two 'conventional' designs.
T
Taper chargers are supplied as single- step (Wa profile) or two-step (WoWa profile) variants, and are available for connection to single-phase and three- phase electrical supplies. This charger is only capable of operating with a single charging profile, and its recharge time is dependant upon the stability of the electrical supply voltage. If the AC voltage reduces the AC input current goes up, and the DC output current goes down.
The lower running costs mean High Frequency chargers can usually repay their slightly higher initial acquisition costs over a five-year lift truck operating lifecycle.
Taper chargers are significantly challenged by over-discharged (i.e. >80% discharge depth) batteries. The high initial current requirement strains the charger's rectifier and transformer leading to possible overload and failure. Another limitation of this charger type is the fact that its termination methods are often less sophisticated, for example in terms of being able to detect when the battery is fully charged. This makes the Taper charging process less efficient than other technologies. For this and other reasons, Taper-type charger technology is now at the end of its
24 ShD August 2010
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working lifespan, and is likely to be increasingly phased out during the next few years.
Regulated improvements Regulated chargers are the other 'conventional' approach, providing the benefit of ensuring better protection for themselves, and to a degree the battery, from unintentional abuse. Unlike Taper chargers, Regulated chargers employ a thyristor-controlled rectifier, and will frequently provide multiple charging profiles. However, methods of termination vary and to get the best results it is necessary to determine the needs of individual battery technologies, e.g standard wet batteries, low-maintenance or VRLA (valve-regulated lead-acid). However it is possible to calculate recharge time more specifically, since by virtue of their design, Regulated chargers are able to compensate for AC power supply variations while charging takes place. There is also a much lower risk of component failure due to over- discharge, since the Regulated charger can limit the DC output, and the load placed upon vital components.
High-frequency chargers take the spotlight
Regulated chargers are still used widely but the main focus of interest is now in High Frequency chargers. These have been available for approximately 20 years, but have evolved considerably over that time.
Higher efficiency is their principal benefit, due to the fact they can use smaller, more efficient components with lower losses. While precise figures vary between models, typically High Frequency chargers require 5% to 10% less power per battery recharge. In turn,
AC input currents are approximately 30% lower than those drawn by Regulated chargers.
The resulting lower running costs (in terms of electricity charges) mean High Frequency chargers can usually repay their slightly higher initial acquisition costs over a five-year lift truck operating lifecycle. And, of course, by consuming less power, High Frequency chargers are also a 'greener' option, which is an increasingly important consideration for all modern businesses.
Smaller dimensions are another benefit. In many cases the charger is 25% the size of a conventional design, and weighs considerably less. This greatly reduced footprint, in turn, permits the creation of new concepts in battery changing and handling systems. Reducing the overall size of a system makes access and installation easier, and provides savings in material and labour. Like Regulated chargers, High Frequency chargers will protect operators by reducing the risk of explosion and injury if accidental disconnection under load takes place. Their reliability, similarly, is assured by compensating for varying AC inputs and over-discharge situations. And while they have the Regulated chargers' profile capability, on top of that High Frequency chargers can link with other systems such as battery queuing and storage networks where data can be accessed remotely or on site. Finally, depending upon their display type and physical size, most high frequency chargers will retain an operational history that can indicate where battery abuse is taking place. End users can employ this to rectify problems and protect the life expectancy of an often significant investment.●
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opinion
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